The present application is based on and claims priority from Japanese Patent Application 2000-238621 filed Aug. 7, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a voltage regulator of a vehicle AC generator for controlling current supplied to a field coil of the generator by a semiconductor element.
2. Description of the Related Art
Japanese patent publications P2529237 and P3019377, which was filed by this applicant, disclose arts (hereinafter referred to as the turn-off moderation technology) for reducing electromagnetic noises caused by inductance of a power line that connects a semiconductor elements and a battery when the semiconductor element is turned off to cut current supplied to a field coil.
In this turn-off moderation technology, the control voltage or a switching capacity of the semiconductor element is changed at a constant rate so that current-reduction rate of the semiconductor element can be made smaller than the current-reduction rate of the semiconductor element whose control voltage is changed stepwise.
However, the disclosed turn-off moderation technology, a large amount of internal loss may be caused during turnoff transition period of the semiconductor element if the current reduction rate lowers. Since it is necessary to increase the surface area of the semiconductor element and to improve the cooling structure thereof, such turn-off moderation technology has not been put into practice.
In particular, a large amount of heat is generated during the former stage of the turn-off transition period due to a large inductance of the field coil.
The present invention has been made in view of the above stated problem of the conventional technology.
Therefore, a main object of the invention is to provide a voltage regulator of a vehicle AC generator, in which heat generated by a semiconductor element when current supplied to the field coil is reduced and noises are suppressed.
A voltage regulator of a vehicle AC generator according to a feature of the invention is comprised of a semiconductor element for controlling current supplied to a field coil of the AC generator, a flywheel element connected in parallel with the field coil, a voltage regulating circuit that controls the semiconductor element to turn on or off, and a switching capacity control circuit for controlling a current-switching capacity of the switching element during a former stage of a turn-off transition period of the semiconductor element to be larger than that during the latter stage of the turn-off transition period.
Accordingly, comparing with an ordinary turn-off operation that reduces current switching capacity stepwise during the former stage of the turn-off transition period, the current change rate between the main electrodes can be reduced to be less thereby to increase a period for reducing the turn-on current of the semiconductor element to zero, so that surge voltage and electromagnetic wave, which are proportional to current change rate, can be reduced.
Comparing a conventional turn-off moderation technology that maintains the switching capacity reduction rate (which is the same as the current reduction rate of the semiconductor element) constant from the former stage to the last stage of the turn-off transition period, the necessary turn-off transition period can be shortened. Therefore, not only heat generation can be reduced but also frequency components of electromagnetic noises that are inversely proportional to the turn-off transition period can be changed. As a result, electromagnetic noises that are harmful to electronically controlled on-vehicle devices can be suppressed.
In order to change the current of the semiconductor element from a turn-on current to zero in an ideal step shape, the surge voltage of the battery power line is related to a product of the differentiated value of current change and wire inductance of battery power line. The magnitude of the surge voltage becomes larger as the frequency increases. On the other hand, AC impedance (1/jxcfx89c) at the space around the battery power line for each frequency component of the surge voltage decreases as the frequency increases. Therefore, the magnitude of the electromagnetic noises radiated outward from the battery power line when the semiconductor element turns off increases in proportion to a square of the frequency. Thus, the magnitude of the electromagnetic noises becomes much larger when the current of the semiconductor element is turned off stepwise. However, since the turn-off transition period is very short, heat generation during the turn-off transition period is very small.
If the current switching capacity, which depends on voltage applied to a control gate of the semiconductor element, is reduced at a fixed reduction rate, the current reduction rate during the turn-off transition period decreases in inverse proportion to increase in the turn-off transition period. Therefore, the electromagnetic noises can be very much reduced. However, the heat generation by the semiconductor element during the turn-off transition period increases that much.
According to this feature, a medium characteristic of both turn-off technologies can be provided, so that the electromagnetic noises can be suppressed without large heat generation.
In other words, the electromagnetic noises generated during the turn-off transition period are suppressed much more than the case of the stepwise change, and mean value of the current during the turn-off transition period and the length of the turn-off transition period can be reduced to less than the turn-off moderation technology.
Moreover, according to this feature, increase in the electromagnetic noises can be effectively suppressed, as described below, although the current switching capacity is reduced during the former stage of the turn-off transition period, and although the current does not decrease very much and the voltage drop across the main electrodes of the semiconductor element is very large, which otherwise causes large heat because of the accumulated magnetic energy of the field coil.
According to another feature of the invention, the drive means changes control voltage or control current of the semiconductor element at an approximately fixed change rate during the former stage of the turn-off transition period and the latter stage, and the change rate at the former stage is set larger than the change rate at the latter stage.
According to this feature, the current switching capacity can be changed by a simple circuit. In the meantime, the control voltage or the control current is the potential or the current of the control gate of the semiconductor element.
According to another feature of the invention, the semiconductor element carries out a follower operation. The follower operation is operation of a circuit in which the field coil and the flywheel diode are connected to the source electrode or the emitter electrode.
In case of the N-channel MOSFET, because the inductance of the field coil in a source follower circuit is very large, when the gate voltage of the semiconductor element decreases during the former stage of the turn-off transition period, magnetic energy of the field coil is discharged to maintain the current supplied to the field coil, thereby lowering the potential of the source electrode.
The reduction of the source voltage suppresses the current change of the semiconductor element, because the channel current of the N-channel MOSFET is proportional to a square of the voltage Vgs between the gate and the source in the saturation range. On the other hand, reduction of the source electrode potential causes increase in voltage effect of the N-channel MOSFET or the semiconductor element and in heat generation.
At the former stage of the turn-off transition period, although the current switching capacity of the semiconductor element decreases due to decrease in the gate electrode potential, current flows in the semiconductor element to restrict the current change due to discharge of the magnetic energy. Thus, the channel current becomes large during the former stage of the turn-off transition period, and the voltage drop becomes large and heat generation becomes large at the semiconductor.
In the former stage of the turn-off transition period during which the source electrode potential of the semiconductor element changes from a high power source voltage to a low power source voltage, magnetic energy of the field coil is discharged. Therefore, change in the channel current is small, the current reduction rate is small, and the electromagnetic noises are small although, as stated above, the absolute value of the current is large and heat generation is large. Therefore, it is preferable to lower the gate electrode voltage, i.e. the control voltage as quickly as possible, thereby to shift to the next current reduction process.
Even if the quick shifting is carried out and the gate potential of the source follower N-channel MOSFET is quickly lowered, the electromagnetic noises will not increase very much since the current change at the former stage is small as stated above.
If the source electrode potential of the semiconductor element becomes about xe2x88x920.7 V or 0.7 V lower than the low power voltage or a ground voltage, return current is supplied from the flywheel diode to the source electrode of the N-channel MOSFET (semiconductor element). Consequently, the channel current of the N-channel MOSFET corresponding to the sum of the amount of the return current and loss of the magnetic energy decreases as the control voltage decreases. This decrease in the channel current causes the electromagnetic noises. Therefore, it is preferable that the reduction rate of the control voltage or reduction rate of the current switching capacity of the N-channel MOSFET is controlled to reduce the electromagnetic noises. As a result, increase in the electro-magnetic noises and heat generation can be controlled. The above explanation is given as to the source follower operation. However, it can be given as to the emitter follower operation in substantially the same manner. A source-grounded operation is also similar to the above.
In a circuit in which the drain electrode or the collector electrode is connected to the field coil and the flywheel diode, if the gate electrode potential is lowered to lower the current switching capacity of the transistor, the field coil discharges magnetic energy in the form of current in order to maintain the current as it has been. Consequently, the drain electrode potential increases quickly.
The increase in the drain electrode potential means the increase in the voltage drop across the main electrodes of the semiconductor element by a large amount of the channel current. Therefore, the heat generation of the semiconductor element or the product of channel current and the voltage drop sharply increases. The increase in the drain electrode potential further increases channel current because of increase in the drain""s depletion layer in the channel region.
In summary, in the source-grounded circuit, it is also necessary to quickly lower the control voltage (gate electrode voltage) or the current switching capacity of the semiconductor, thereby stopping a large amount of heat generation during the former stage of the turn-off transition period in which the current change becomes smaller.
If the drain electrode potential becomes 0.7 V or more higher than the higher power voltage, return current flows through the flywheel diode so that the drain electrode potential can be prevented from rising further. At the same time, the channel current decreases by the amount corresponding to the sum of the return current and the loss of the magnetic energy. Therefore, the reduction rate of the current switching capacity is reduced to suppress the electromagnetic noises that are generated in proportion to this reduction rate.
According to another feature of the invention, the voltage regulator further includes a comparator for comparing a signal voltage related to a voltage drop of main electrodes of the semiconductor element with a reference value, wherein the drive means decreases a decrease rate of current switching capacity of the control electrode of the semiconductor element to a smaller value right after the signal voltage becomes the reference value during the turn-off transition period.
Thus, it is possible to detect the former stage of the turn-off transition period in which the current reduction rate is to increase and the latter stage of the turn-off transition period in which the current reduction rate is decrea.
The signal voltage is any one of the voltage at the joint of the semiconductor element and the field coil, the voltage drop across the main electrodes of the semiconductor element, the voltage drop by the field coil, the current flowing through the main electrodes of the semiconductor element, the current of the flywheel element and the current of the field coil. In particular, the return current is more preferable because the return current detected in the source follower circuit or the emitter follower circuit means start of the reduction in the current of the semiconductor element.
According to another feature of the invention, the comparator increases charging current of the gate electrode of the semiconductor element, right after the signal voltage becomes the reference value during the turn-on transition period of the semiconductor element; and the reference value for changing the charging current during the turn-on transition period is set higher than a predetermined value for changing the charging current during the turn-off transition period.
Even if the detection of the signal voltage delays, the control voltage applied to the semiconductor element does not lower quickly to turn off the semiconductor element immediately. Therefore, the change speed of the current flowing through the semiconductor element can be gradually lowered, so that the spike voltage and radio noises can be reduced effectively.
According to another feature of the invention, a voltage regulator of a vehicle AC generator is comprised of a semiconductor element for controlling current supplied to a field coil, a flywheel element connected in parallel with the field coil, generation voltage regulating circuit for controlling the semiconductor element to turn on or off so that an output voltage of the regulator can be regulated to a predetermined control voltage according to a signal that is related to the output voltage, switching capacity control circuit including a charge pump voltage boosting circuit that supplies the semiconductor element operating follower operation with a higher gate voltage than the output voltage, and an operation control circuit for stopping the charge pump circuit from a former stage of turn-off transition period of the semiconductor element.
According to this feature, the driving current of the charge pump circuit and the accompanied noises thereof and the noises of the AC generator can be reduced. The charge pump boosting circuit can be stopped during the turn-off transition period and the post cut-off state maintaining period. In addition, it can be stopped while the semiconductor element fully turns on and the voltage across the main electrodes thereof is low, with the same effect.
By the charge pump circuit, excessive electric charge is not charged at the gate electrode of the semiconductor element. Therefore, only a small amount of electric charge is necessary during the former stage of the turn-off transition period. This shortens the duration of the former stage and suppresses the heat generation.
According to another feature of the invention, a voltage regulator of a vehicle AC generator is comprised of a semiconductor element for controlling current supplied to a field coil of the vehicle AC generator having a rectifier for rectifying generated AC output, a flywheel element connected in parallel with the field coil, generation voltage regulating means for controlling the semiconductor element to turn on or off so that an output voltage of the regulator can be regulated to a predetermined control voltage according to a signal that is related to the output voltage, and a switching capacity control circuit. The rectifier is characterized by comprising a diode formed of a diode of a short recovery time, the maximum current change rate (%) of the semiconductor element during the recovery period is less than twice as long as the maximum current change rate (%) during turn-off transition period or turn-on transition period of the semiconductor element.
When the semiconductor element turns off or turns on, while the return current flows, or while the AC output power is rectified by the rectifier, when the bias voltage of the diode changes from forward to backward, the recovery current of the diode can be controlled. Therefore, the spike voltage appearing on the charging line can be reduced so that the radio noises can be suppressed.
In addition, the change rate per unit time of the current of the semiconductor element when turning on is made less than twice as much as the current change rate of the recovery current of the diode of the rectifier. Therefore, the spike noise level on the charging line caused by the switching can be controlled at the diode-return current noise level, so that the spike voltage and the radio noises of the AC generator can be effectively suppressed.
In addition, the back recovery time shortening type diode has a lower breakdown voltage, and the breakdown voltage of the flywheel element can be set to be lower. Therefore, the recovery voltage of the flywheel element can be lowered, so that noises can be effectively suppressed.